The fabrication of current all-ceramic dental restorations often requires extensive labor and time and the proficiency of highly skilled technicians Many state-of-the-art dental restorations reveal a sense of artistry that can typically only be achieved manually or “by hand.” While aesthetics are preserved with this process, microstructural inhomogeneities may appear, affecting strength and reliability. The industry has attempted to automate this process by, for example, pressing crowns. Although pressable crowns reduce the time required to produce a crown, about two hours of concerted effort is necessary to complete a crown. Pressed crowns may also suffer from similar strength and relability problems typical of “hand made” crowns.
Computer assisted design/computer assisted milling (CAD/CAM) processes and equipment have been recently introduced into the dental industry. In these processes a three-dimensional image of a stump of a tooth is created along with the teeth surrounding the stump in an effort to create a dental restoration which is to be placed over the stump. This image is displayed on a computer screen. Based on the stump and surrounding teeth, the dental technician may then select a tooth from a plurality of tooth forms stored in the computer to best fit the stump. The selected tooth is projected onto the stump until an optimum positioning and fit of the dental restoration is achieved. The digital data concerning the dental restoration thus formed are supplied to a numerically controlled milling machine operating in three dimensions. The milling machine cuts a blank of metal or porcelain material into the dental restoration design based on the data supplied.
U.S. Pat. No. 4,663,720 to Duret and commonly assigned U.S. Pat. No. 5,775,912 to Panzera et al. each teach CAD/CAM systems and materials which are designed to reduce labor and increase reliability and are herein incorporated by reference. U.S. Pat. No. 5,775,912 is directed to a method of making a dental restoration using soft-sintered porcelain pellets. The method requires the step of investing the tooth structure with an investment refractory material prior to fusing and fully densifying because the glass-ceramic will begin to flow during this step. The investment refractory material provides a mold to maintain the shape of the glass-ceramic during sintering.
U.S. Pat. No. 5,910,273 to Thiel teaches a process for the manufacture of dental materials using CAD/CAM methods wherein a porosity-sintered blank is milled to a desired shape. In order to densify the material, it must be infiltrated with a glass material.
CAD/CAM and copy milling systems designed for the dental industry by Vita Celay, Siemens and Nobelpharma have also been shown to reduce labor. However, some of the materials used in these systems have been shown to be weak or unaesthetic. Moreover, it has been observed, that the use of fully fused dental porcelain pellets wear down cutting and milling tools and significantly slow down the process of dental restoration fabrication. The milling of fully fused dental porcelains may result in excessive chipping and flaking, thus affecting the precision of the milling operation and ultimately the fit between the restoration and the patient's natural teeth.
There is a need to provide materials for use in CAD/CAM operations that are strong and aesthetically pleasing. It is desirable to provide materials for use in CAD/CAM operations that reduce wear of cutting tools on milling machines.